# std::atomic_fetch_add, std::atomic_fetch_add_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
template< class T > T atomic_fetch_add( std::atomic<T>* obj, typename std::atomic<T>::difference_type arg ) noexcept; | (1) | (since C++11)
template< class T > T atomic_fetch_add( volatile std::atomic<T>* obj, typename std::atomic<T>::difference_type arg ) noexcept; | (2) | (since C++11)
template< class T > T atomic_fetch_add_explicit( std::atomic<T>* obj, typename std::atomic<T>::difference_type arg, std::memory_order order ) noexcept; | (3) | (since C++11)
template< class T > T atomic_fetch_add_explicit( volatile std::atomic<T>* obj, typename std::atomic<T>::difference_type arg, std::memory_order order ) noexcept; | (4) | (since C++11)

Performs atomic addition. Atomically adds arg to the value pointed to by obj and returns the value obj held previously. The operation is performed as if the following was executed:

1,2) obj->fetch_add(arg)
3,4) obj->fetch_add(arg, order)

If `std::atomic<T>` has no `fetch_add` member (this member is only provided for integral, floating-point(since C++20) and pointer types except bool), the program is ill-formed.

### Parameters

obj | - | pointer to the atomic object to modify
arg | - | the value to add to the value stored in the atomic object
order | - | the memory synchronization ordering

### Return value

The value immediately preceding the effects of this function in the modification order of *obj.

### Example

Single-writer/multiple-reader lock can be made with `std::atomic_fetch_add`. Note that this simplistic implementation is not lockout-free.

Run this code

    #include <atomic>
    #include <chrono>
    #include <iostream>
    #include <string>
    #include <thread>
    #include <vector>

    using namespace std::chrono_literals;

    // meaning of cnt:
    //  5: readers and writer are in race. There are no active readers or writers.
    //  4...0: there are 1...5 active readers, The writer is blocked.
    // -1: writer won the race and readers are blocked.

    const int N = 5; // four concurrent readers are allowed
    std::atomic<int> cnt(N);

    std::vector<int> data;

    void reader(int id)
    {
        for (;;)
        {
            // lock
            while (std::atomic_fetch_sub(&cnt, 1) <= 0)
                std::atomic_fetch_add(&cnt, 1);

            // read
            if (!data.empty())
                std::cout << ("reader " + std::to_string(id) +
                              " sees " + std::to_string(*data.rbegin()) + '\n');
            if (data.size() == 25)
                break;

            // unlock
            std::atomic_fetch_add(&cnt, 1);

            // pause
            std::this_thread::sleep_for(1ms);
        }
    }

    void writer()
    {
        for (int n = 0; n < 25; ++n)
        {
            // lock
            while (std::atomic_fetch_sub(&cnt, N + 1) != N)
                std::atomic_fetch_add(&cnt, N + 1);

            // write
            data.push_back(n);
            std::cout << "writer pushed back " << n << '\n';

            // unlock
            std::atomic_fetch_add(&cnt, N + 1);

            // pause
            std::this_thread::sleep_for(1ms);
        }
    }

    int main()
    {
        std::vector<std::thread> v;
        for (int n = 0; n < N; ++n)
            v.emplace_back(reader, n);
        v.emplace_back(writer);

        for (auto& t : v)
            t.join();
    }

Output:

    writer pushed back 0
    reader 2 sees 0
    reader 3 sees 0
    reader 1 sees 0
    <...>
    reader 2 sees 24
    reader 4 sees 24
    reader 1 sees 24

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
P0558R1 | C++11 | exact type match was required because `T` was deduced from multiple arguments | `T` is only deduced from obj

### See also

fetch_add | atomically adds the argument to the value stored in the atomic object and obtains the value held previously (public member function of `std::atomic<T>`)
atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) | subtracts a non-atomic value from an atomic object and obtains the previous value of the atomic (function template)
C documentation for atomic_fetch_add, atomic_fetch_add_explicit

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